Known co-extrusion processes involve melting of at least two separate polymer compositions and their simultaneous extrusion and immediate combination. The extrudate can be cooled until the polymers have solidified and can be mechanically wound onto a roll. Winding the extrudate around a chilled roll may accelerate the cooling. The extrudate may be oriented to a controlled degree in the machine and/or transverse direction. This drawing may be performed at temperatures below the melting point of the co-extrudate. In this way, articles can be made combining the desired properties of different polymer compositions.
Co-extruded films are generally made from polymer compositions, which develop considerable mechanical strength upon cooling by the forming of crystalline phases. Such polymer compositions are also capable of developing increased strength upon orientation of the compositions and better alignment of the crystalline regions.
Elasticity in films is desired for a number of applications. Examples of such applications are in personal care products, such as diaper back sheets, diaper waistbands, and diaper ears; medical applications, such as gowns and bags; and garment applications, such as disposable wear. In use in the final structure, elastic articles can provide desirable characteristics, such as helping to achieve compliance of garments to an underlying shape. In diaper waistbands, for example, a high elastic recovery ensures the return of the waistband to its original shape throughout the use of the diaper.
Elasticity is generally obtained from the use of amorphous elastomeric polymer compositions. There are, however, many difficulties and problems associated with the processing of such polymer compositions into articles such as films and fibers. For example, elasticity limits the line speed, particularly during processing at high line speeds, because the tension applied to the film causes the film to extend, sometimes in an unstable manner.
Furthermore, elastic polymers are generally high molecular weight amorphous polymers that can be difficult to process into articles such as films, fabrics and fibers. Another difficulty in processing elastic films arises from the tackiness of the films on the roll, which causes “blocking”, i.e., sticking of the film to itself. This limits the storage of the article after it has been produced. Elastic polymers can also have poor aesthetics, including, for example, poor surface appearance and a rubbery or tacky feel or touch.
Several approaches have been taken to alleviate these problems. U.S. Pat. No. 6,649,548 discloses laminates of nonwoven fabrics with films to impart a better feel. U.S. Pat. Nos. 4,629,643 and 5,814,413 and PCT Publications WO 99/47339 and WO 01/05574 disclose various mechanical and processing techniques used to emboss or texture the film surface in order to increase the surface area and improve the feel. U.S. Pat. Nos. 4,714,735 and 4,820,590 disclose films comprising an elastomer, ethylene vinyl acetate (EVA), and process oil that are prepared by orienting the film at elevated temperature and annealing the film to freeze in the stresses. The film is subsequently heated, which shrinks and forms an elastic film.
In one embodiment, these references also disclose films having layers of ethylene polymers or copolymers on either side of the elastic film to reduce tackiness. By heat-setting the film, it can be stabilized in its extended condition. Upon application of heat higher than the heat setting temperature, the heat set is removed and the film returns to its original length and remains elastic. Two heating steps are involved, adding cost and complexity. U.S. Pat. No. 4,880,682 discloses a multilayer film comprising an elastomer core layer and thermoplastic skin layer(s). The elastomers are ethylene/propylene (EP) rubbers, ethylene/propylene/diene monomer rubbers (EPDM), and butyl rubber, in a laminated structure with EVA as the skin layers. After casting, these films are oriented to yield films having a micro-undulated surface providing a low gloss film.
Micro-textured elastomeric laminated films having at least one adhesive layer are disclosed in U.S. Pat. Nos. 5,354,597 and 5,376,430. U.S. Pat. No. 4,476,180 describes blends of styrenic block copolymer based elastomers with ethylene-vinyl acetate copolymers to reduce the tackiness without excessively degrading the mechanical properties.
WO 2004/063270 describes an article that includes a low crystallinity layer and high crystallinity layer capable of undergoing plastic deformation upon elongation. The crystallinity layer includes a low crystallinity polymer and, optionally, an additional polymer. The high crystallinity layer includes a high crystallinity polymer having a melting point at least 25 C higher than that of the low crystallinity polymer. The low crystallinity polymer and the high crystallinity polymer can have compatible crystallinity.